Support structure for a superconducting magnet

ABSTRACT

An electrical magnet having a support structure which comprises a flat plate of structural material and abutment means engaging the conductor of the electrical magnet to the plate. The abutment means may comprise a groove in a face of the plate which receives and restrains the conductor, a raised flange on a face of the plate which forms such a groove, or a flange which extends around and is affixed to the periphery of the plate and which is formed to fit tightly over a coiled conductor. When current is flowing in the conductor of the magnet, all points on the conductor will experience a magnetically induced force, one component of which is in an outward direction parallel to the plane of the plate. The abutment means opposes this component of the force and transmits it to the plate, which is consequently stressed in tension in two dimensions. Several individual magnets supported by the support structure may be combined to form large magnets having solenoidal, toroidal or other configurations. The magnets may be built with a normal or a superconducting electrical conductor.

1221 Filed:

' United States Patent 1191 Young et al.

[ SUPPORT STRUCTURE FOR A SUPERCONDUCTING MAGNET [75] Inventors: Warren C. Young; Roger W. Boom;

Harold A. Peterson, all of Madison, Wis.

[73] Assignee: Wisconsin Alumni Research 1152 US. Cl 335/216; l74/DIG. 6

[51] Int. CI. HOIF 7/22 [58] Field of Search 335/216, 282, 299; 336/195, 200; 174/DIG. 6

[56] References Cited UNITED STATES PATENTS 2,583,854 1/1952 Kehbel 336/200 x 2.584.592 2/1952 Kehbel 336/200 x 2,982,888 5/1961 Whearley.. 335/299 2,982,889 5/1961 Whearley 335/299 3,130,355 4/1964 Younger 335/299 3.177.408 4/1965 M1115 et al. 335/216 x 3,283,217 11/1966 Cherry 335/216 UX FOREIGN PATENTS OR APPLICATIONS l,532,l96 7/l967 France l74/DIG. 6

[ Nov. 11, 1975 Primal E.1't1minerG. Harris Attorney, Agent, or Firt7t-Theodore J. Long; John M. Winter; Harry C. Engstrom [5 7 ABSTRACT An electrical magnet having a support structure which comprises a flat plate of structural material and abutment means engaging the conductor of the electrical magnet to the plate. The abutment means may comprise a groove in a face of the plate which receives and restrains the conductor, a raised flange on a face of the plate which forms such a groove, or a flange which extends around and is affixed to the periphery of the plate and which is formed to fit tightly over a coiled conductor. When current is flowing in the conductor of the magnet, all points on the conductor will experience a magnetically induced force, one component of which is in an outward direction parallel to the plane of the plate. The abutment means opposes this component of the force and transmits it to the plate,

which is consequently stressed in tension in two dimensions. Several individual magnets supported by the support structure may be combined to form large magnets having solenoidal, toroidal or other configurations. The magnets may be built with a normal or a superconducting electrical conductor.

5 Claims, 8 Drawing Figures U.S. Patent Nov. 11, 1975 SUPPORT STRUCTURE FOR A SUPERCONDUCTING MAGNET BACKGROUND OF THE INVENTION 1. Field of the Invention I This invention pertains generally to thefield of electrical magnets and inductors and support structure therefor.

2. Description of the Prior Art It is a basic principle of physics that a conductor carrying electric current in a magnetic field experiences a force that is proportional to the product of the magnitudes of the current and the magnetic field. Self in duced magnetic forces are impressed on inductors having one or more turns of conductor, such inductors and any associated support structure being denoted generally herein as electrical magnets.

A simple solenoid coil can be considered as an example of an electrical magnet subject to magnetic forces. Each of the many circular turns in the coil will be subject to a component of magnetic field whose direction will be perpendicular to the plane of the circular turn. As a result, each point on a circular turn will experience an outward radial force which must be opposed by a tensile stress in the conductor or by an external support structure. The problem of containing these forces is particularly acute with superconducting magnets and inductors because of the enormous currents that can be carried by superconductors of small dimension.

The commonly accepted support structure for a solenoid coil consists of an external cylinder or hoop of structural material which fits over the coil. With this method of support the radial forces on the conductor in the coil are opposed by tension forces in the conductor and tension forces in the external support cylinder. The tensile stresses on each small element of material in the support cylinder will be primarily in only one dimension (along a tangent to the circular cylinder at each point) for the case of a thin walled cylinder. This results in a much less than optimal utilization of the structural material, a situation that would be achieved if each small three-dimensional element of material in the support structure were stressed equally in all three dimensions.

It is apparent that the conductor of a solenoid coil and an external support cylinder in intimate contact therewith must undergo approximately the same magnitude of strain as the coil expands radially under the influence of a magnetic field. The strain in the support cylinder is, of course, directly proportional to the stress which the support cylinder experiences up to the proportional limit of the cylinder material. The thin walled support cylinder experiences stress in only one dimension and will thus be strained in that dimension much more than it would be if in addition, tensile stresses were also applied to the support structure in one or both of the orthogonal dimensions. It is important to minimize strain in ordinary non-superconductive coils .because the resistance of copper or aluminum or other conductor increases with cold work and because of the possibility of breakage of the conductor itself or of its insulation. It is even more important to minimize strain in composite superconductors, which typically consist of a superconducting material such as niobiumtitanium embedded in a normal conductor such as oxygen free high conductivity (OFI-IC) copper. The thermal and electrical contact between the normal conductor and the superconductor and the high conductivity of the normal conductor are critical to proper functioning of the composite superconductor. These properties are degraded by excessive strain, and such strain may also result in breaks or cracks in the superconductor.

The forces described above for solenoid coil configurations also are present in other coil configurations such as toroids, dipoles, quadrupoles, and octupoles. Support structures will thus be required for coils in these configurations when large magnetic fields are applied thereto.

SUMMARY OF THE INVENTION We have invented a support structure for electrical magnets and other inductors which maintains the strain in the electrical conductors of such magnets at an acceptable level, but requires much less structural material than present known magnet support structures. Our magnet support structure is composed of one or more magnet support units which can be combined together and with a conductor to form magnets of widely varying configurations such as solenoids and toroids.

The magnet support units can be built to support the individual conductor turns in any desired planar geometry, as, for example, a circular or rectangular shape.

The magnet support unit has a substantially flat plate of structural material which is sized and shaped so that the periphery of the plate conforms substantially to the outer periphery of the turns of conductor that are to be supported. The turns of electrical conductor may be engaged to the plate by means of a spiral shaped groove or grooves formed in one or both of the two opposite faces of the plate. The conductor is inserted into the groove to form a spiraled coil and is effectively restrained from movement in the plane of the plate by the abutting walls of the groove. Such a groove may also be formed by the walls of a flange raised from and afflxed to the faces of the plates. Another means of engaging coil turns to the plate utilizes a flange which is attached to and extends around the periphery of the plate. The flange is shaped to fit tightly over and circumscribe a plurality of coil turns which have been pre-formed in the desired coil shape. The inner wall of the flange abuts with and restrains outward movement of the coil turns in the plane of the plate. The flange may also be used in forming a coil in another manner by winding a conductor around the outside of the flange and then securely attaching the conductor to the flange.

When in use, the coil conductors associated with each support unit will experience a component of magnetic field which will be oriented in a direction normal to the plane in which the plate of the support unit lies. Thus, any forces exerted on the conductors because of this component of the magnetic field will be exerted parallel to the plane of the plate and will be opposed by the walls of the groove, or in the alternative configuration, by the flange on the plate. The plate will necessar ily be stressed in two dimensions. For a given amount of structure less stress is applied in any single direction than is the case with present known support structures, which are stressed in only one direction at each point in the structure. Our magnet support units can thus contain the same magnetically induced forces with much less structural material because the stress is distributed in two dimensions.

Support structures for large, high field intensity magnets can be built using our magnet support units by attaching the units together to form the desired coil con-.

figurations. For example. a solenoid can be formed by "drawings showing preferred embodiments of a magnet support unit and electrical magnets formed therefrom exemplifying the principles of our invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an axial elevation view of a preferred embodiment of a magnet support unit exemplifying the principles of our invention.

FIG. 2 is a cross-sectional view taken along the line 22 of FIG. 1.

FIG. 3 is a perspective view of a plurality of the magnet support units of FIG. 1 combined together and with electrical conductors to form a solenoidal'electrical magnet.

FIG. 4 is a perspective view of a plurality of the magnet support units of FIG. 1 combined together and with electrical conductors to form a toroidal electrical magnet.

FIG. 5 is an axial elevation view of another embodiment of a magnet support unit exemplifying the principles of our invention.

FIG. 6 is a cross-sectional view taken along the line 66 of FIG. 5.

FIG. 7 is a perspective view of the magnet support unit of FIG. 5 shown combined with a conductor to form an electrical magnet.

FIG. 8 is a perspective view of the magnet support unit of FIG. 5 shown with electrical conductor wound around and attached to the periphery thereof to form an electrical magnet.

DESCRIPTION OF THE PREFERRED EMBODIMENT terial. As best shown in FIG. 1, the flange 13 generally spirals in from the periphery 11a of the plate, with the walls of the flange defining a continuous groove 13a. The cross-section of the groove 13a is preferably rectangular in shape, as shown in FIG. 2, with the walls of the flange being substantially perpendicular to the plane of the plate 11. The groove 13a may have the general shape of any closed plane figure, but will preferably spiral in from the outer periphery 11a to form multiple continuously connected turns. As best shown in FIG. 1, a hole 14 is formed in the plate 11 at the innermost ends of the grooves 13a to put the grooves on the opposite faces of the plate in communication. The grooves 13a can be formed in an alternative manner by forming a substantially rectangular channel in the flat faces 12 of the plate, with this channel or groove also spiraling .in from the outer periphery of the plate. It is preferable that the plate 11 be made of a non-magnetic structural material such as stainless steel or epoxy fiberglass to minimize the magnetic interaction of the plate structure with ambient magnetic fields.

An electrical magnet can be formed using our magnetic support unit 10 by combining it with an electrical conductor 15 which is inserted into the groove 13a on one face of the plate, passed through the hole 14 and inserted in the groove 13a on the opposite face of the plate. In order for current to be flowing in the same angular direction in the conductor 15 on both sides of the plate, it is apparent that it is necessary for the grooves 13a on the opposite faces of the plate to form spirals that turn in opposite directions; The walls of the groove 13a abut with the conductor 15 to restrain movement of the conductor in a direction parallel to the plane in which the plate 11 lies that is, a plane parallel to and between the substantially parallel faces 12 of the plate. Current flowing in the conductor 15 will produce a component of magnetic field which will be oriented normally to the plane of the plate 11. The resulting force on each small portion of the conductor 15 from this component of magnetic field will be directed normal to the conductor in the plane of the plate and will thus be opposed by the abutting walls of the groove As shown in FIG. 3 a plurality of our magnet support units 10 can be combined to form a support structure for a large solenoid magnet 16. This support for the solenoid magnet 16 is formed by stacking the magnet support units 10 one atop another in a straight line and attaching them together, while the conductors l5 associated with the magnet support units are'electrically connected together to form a continuous conducting path through the solenoid. The magnet support units 10 will preferably be stacked closely together when in use to maximize the aggregate magnetic field experienced by the conductors in each support unit.

As shown in FIG. 4, an electrical magnet 17 having a toroidal configuration can be formed from a plurality of our magnet support units stacked together in a circle. The toroidal electrical'magnet 17 is shown in FIG. 4 with a few magnet support units 10 removed to illustrate the placement ofthe support units. The electrical conductors 15 in each support unit in the toroidal magnet 17 are also connected together to form a continuous conducting path through the magnet. Although the magnet support units 10 have been shown as having a circular periphery in FIG. 4, these support units may also be made with a D shaped periphery which is pref-. erable for forming toroidal shaped magnets.

The magnet support units 10 can be used to support the conductive turns of either a normal or a superconducting magnet. However, special considerations. arise when a superconducting magnet is supported. The conductor 15 in a superconducting magnet would preferably be a composite conductor composed of both a normal conductor such as copper and a superconductor such as a niobium-titanium alloy. Cooling of the superconductor can be accomplished by pumping liquid or supercritical helium through passageways in the normal conductor or by immersing the entire magnet support unit 10 and the conductor 15 into a liquid helium bath. In the former case it is desirable to minimize the heat transfer between the composite conductor 15 and the plate 11. This may be done by making the plate 11 out of a low thermal conductivity-material such asepoxy I copper in a copper-superconductor composite conductor should in general be held belowthe yield point for oxygen free high conductivity (OFHC) copperf This limitation is necessary because of the mechanical'hysteresis energy loss which is increased as the material yields, and because the electrical resistance of OFHC copper increases with cold work. If no stress is applied to the composite conductor in the absence ofa magnetic field, the conductor can be stressed up to a maximum of 12,000 psi to 18,000 psi in tension when a magnetic field is applied. Assuming that the composite conductor 15 and the walls of the groove 13 in the plate 11 are in close contact before application of the field, the material in the plate 11 adjacent to grooves 13a willexperience approximately the same strain as the conductor. Thus, the stress that is carried by the plate material will be approximately equal to the stress carried by the composite conductor multiplied by the ratio of the modulus of elasticity of the plate material to the modulus of elasticity of the composite conductor. For example, with a composite conductor that is primarily copper, and a stainless steel plate, the plate could carry a stress of twice the copper stress since the modulus of elasticity of stainless steel is about twice that of copper.

Because that stress level is far below the yield point of stainless steel, a more efficient use of this material can be obtained by pre-stressing the composite conductor, for example, to 12,000 psi in compression. The composite conductor can now be stressed to 12,000 psi in tension and the steel, which undergoes the same strain change, will carry 48,000 psi in tension. The prestressing of the composite conductor can be accomplished in any appropriate manner as for example, by mechanically stressing the plate 11 in tension before inserting the conductor 15 in the grooves 13a, or by inserting the conductor 15 in the grooves 13a while the plate is at an elevated temperature, with the result that the conductor is compressed as the plate cools down and contracts.

Another embodiment of a magnet support unit is shown generally at 18 in FIGS. 5 and 6. The magnet support unit 18 has a solid plate 19 and a flange 20 which extends around and is affixed to the periphery of the plate. The magnet support unit 18 can be used to provide structural support to an electrical coil 21 which has been pre-formed into the desired coil shape. With a circular coil, the plate 19 will be circular and the flange 20 cylindrical, as shown in FlGS. 5 and 6. The coil 21 fits within, and is surrounded by and engaged with the flange 20 on the support unit 18, as best shown in FIG. 7. The flange 20 will preferably fit tightly over the coil 21 to absorb as much stress as possible. The coil 21 may also be pre-stressed in compression to increase the maximum stress that can be applied to the magnet support unit 18. This pre-stressing can be accomplished with a stainless steel magnet support unit 18 by raising the magnet support unit to a high temperature and expanding it before placing the support unit around the coil 21,- thereby stressing the coil in compression as the magnet support unit cools down and contracts. The magnet support units 18 can also be combined to support large solenoidal coils or toroidal coils in the same manner as illustrated previously for the magnet support units 10.

Another contemplated manner of using our magnet support unit 18 is shown in FIG. 8. An electrical magnet can be supported by winding a conductor 22 around the-outside of the flange 20 and attaching the conductor to the flange by means of braces 23 which cover portions of the conductor and are welded or otherwise secured tothe flange. In thepresence of a magnetic field, the outward forces on the conductor 22 are largely transmitted by the braces 23 to the flange 20 and thence to the plate 19, and thus the entire magnet support unit 18 will be stressed in tension.

While the magnet support units 10 and the magnet support units 18 have been shown supporting coils having substantially circular turns, it is apparent that these support units can be built to support turns of conductor having any planar configuration. For example, the grooves 13a in the magnet support unit 10 could be formed in an inward spiraling rectangular or polygonal shape. Similarly, the magnet support unit 18 could be formed with a rectangular or polygonal periphery to properly support a coil of the same shape.

It is understood that our invention is not confined to the particular construction and arrangement of parts herein illustrated and described but embraces all such modified forms thereof as come within the scope of the following claims:

We claim:

1. An electrical magnet, comprising:

a. a coil of composite conductor formed of a superconducting material and a normal conducting material and having an outer periphery;

b. a substantially fiat plate having two opposite substantially parallel faces and a periphery substantially conforming to the outer periphery of said coil;

0. a flange affixed to and extending around the periphery of said plate, said flange fitting over and restrainably engaging the outer periphery of said coil with said coil lying in a plane substantially parallel to the faces of said plate, whereby electrical current passing through said composite conductor, when it is maintained at temperatures at which it is superconducting, will cause said coil to exert outward forces parallel to the faces of said plate on said flange, with said flange transmitting such forces to said plate whereby said plate is stressed in two dimensions; and

d. said plate and said flange thereon being formed of a structural material capable of carrying such forces exerted by said coil when said composite conductor is maintained at temperatures at which it is superconducting.

2. The electrical magnet as specified in claim 1 wherein said flange is formed continuously with said plate, and said flange and plate are formed of a nonmagnetic structural material.

3. The electrical magnet as specified in claim 1 wherein the composite conductor of said coil is stressed in compression by said flange when no electrical current is conducted by said conductor.

4. An electrical magnet, comprising:

a. a substantially flat plate having two opposite sub- 1 said. flange, with said flange transmitting such stantially parallel faCC and a periphery: forces to said plate whereby said plate is stressed in b. a flange affixed to and extending around the petwo i i d riphery of said plate; c. a composite conductor formed of a superconduc- 5 ting material and a normal conducting material which is wound around the outside of said flange to form electrical magnet turns; (1; means for securing said composite conductor to alums at whfchllt superconducimg' said flange, whereby electrical current passing 1 5" T -e lecmcal magnet s spajclfied m (flalm through Said composite Conductor when it is main where n said flange 18 formed continuously with said tained at temperatures at which it is s'uperconduc- Plale, and Said flange and Plate are formed of a ting will cause said composite conductor toexert magnetic Structural materialoutward forces parallel to the faces of said plate on y v e. said plate and said flange thereon being formed of a structural material capable of carrying such forces exerted by said composite conductor when said composite conductor is maintained at temper- 

1. An electrical magnet, comprising: a. a coil of composite conductor formed of a superconducting material and a normal conducting material and having an outer periphery; b. a substantially flat plate having two opposite substantially parallel faces and a periphery substantially conforming to the outer periphery of said coil; c. a flange affixed to and extending around the periphery of said plate, said flange fitting over and restrainably engaging the outer periphery of said coil with said coil lying in a plane substantially parallel to the faces of said plate, whereby electrical current passing through said composite conductor, when it is maintained at temperatures at which it is superconducting, will cause said coil to exert outward forces parallel to the faces of said plate on said flange, with said flange transmitting such forces to said plate whereby said plate is stressed in two dimensions; and d. said plate and said flange thereon being formed of a structural material capable of carrying such forces exerted by said coil when said composite conductor is maintained at temperatures at which it is superconducting.
 2. The electrical magnet as specified in claim 1 wherein said flange is formed continuously with said plate, and said flange and plate are formed of a non-magnetic structural material.
 3. The electrical magnet as specified in claim 1 wherein the composite conductor of said coil is stressed in compression by said flange when no electrical current is conducted by said conductor.
 4. An electrical magnet, comprising: a. a substantially flat plate having two opposite substantially parallel faces and a periphery: b. a flange affixed to and extending around the periphery of said plate; c. a composite conductor formed of a superconducting material and a normal conducting material which is wound around the outside of said flange to form electrical magnet turns; d. means for securing said composite conductor to said flange, whereby electrical current passing through said composite conductor when it is maintained at temperatures at which it is superconducting will cause said composite conductor to exert outward forces parallel to the faces of said plate on said flange, with said flange transmitting such forces to said plate whereby said plate is stressed in two dimensions; and e. said plate and said flange thereon being formed of a structural material capable of carrying such forces exerted by said composite conductor when said composite conductor is maintained at temperatures at which it is superconducting.
 5. The electrical magnet as specified in claim 4 wherein said flange is formed continuously with said plate, and said flange and plate are formed of a non-magnetic structural material. 